Field of the Invention
The present disclosure generally relates to image processing and, more particularly, to an image processing apparatus that can correct an image, an image processing method for the image processing apparatus, and a medium.
Description of the Related Art
When a document is read by using an image reading apparatus (scanner) mounted on a copying machine or a multifunctional peripheral, a problem of “show-through” may occur. The show-through refers to a phenomenon in which, when one side (front side) of the document is read by the image reading apparatus, an image of the other side (back side) of the relevant document is included in the read image. Therefore, “show-through” occurs mainly when a certain image is printed on both sides (front and back sides) of the document to be read by the image reading apparatus. The show-through tends to occur when a high-density image exists on the back side. This phenomenon occurs due to a light amount of a light source when reading an image and a thickness (degree of light transmission) of a medium (paper) of the read document. If the show-through occurs, it becomes hard to recognize each image in the read image, i.e., image quality is degraded.
As a technique for reducing the show-through in a read image, processing for decreasing an overall image density (by intensively applying a “background elimination function”) may be used. In this case, however, not only the density of the show-through but also the density of each image existing on the front side of the document will decrease. Therefore, a low-density image may possibly disappear.
Japanese Patent Application Laid-Open No. 2012-160883 discusses a technique for acquiring a variance value for a fixed range including a target pixel and, when the relevant variance value is equal to or less than a predetermined reference value, performing show-through elimination processing. This processing focuses on a point that a low-density portion in an image is expressed with halftone dots, and utilizes a feature that image data for a region expressed with halftone dots indicates a large variance value. On the other hand, show-through components are hard to be expressed with halftone dots when seen from the front side, and thus indicate a small variance value. Therefore, it is determined whether an image includes the show-through components or is a low-density portion on the front side based on whether the variance value is equal to or less than the reference value, and performs the show-through elimination processing only on the show-through components.
However, the technique discussed in Japanese Patent Application Laid-Open No. 2012-160883 has a problem that a region where the show-through elimination is performed successfully and a region where the show-through elimination is not performed successfully are generated within the same halftone dot region depending on a relation between a halftone dot period of a document and a reading optical resolution of the document.
If the reading optical resolution of a document is determined, a sampling frequency for reading pixels in the document is determined. In this case, if the halftone dot frequency in the document is around one-half of the above-described sampling frequency (around the Nyquist frequency), a region where halftone dots are not correctly resolved is generated in the image data of the read document. Accordingly, one halftone dot region of the document simultaneously includes a region where halftone dots are unresolved and a region where halftone dots are resolved, and is divided into a region having a small variance value and a region having a large variance value. As a result, there occurs a problem that a region where the show-through elimination is performed successfully and a region where the show-through elimination is not performed successfully are generated within the one halftone dot region.